Re: [PATCH] KVM: arm64: Implement KVM_TRANSLATE ioctl for arm64

[David Woodhouse <dwmw2@infradead.org>]

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On Tue, 2025-09-23 at 10:39 +0200, Alexander Graf wrote:
> 
> All modern OSs constrain themselves to only ISV enabled MMIO 
> instructions.

Why? Such does not appear to be required by the architecture, AFAICT?

And what is "MMIO" architecturally anyway? Using LDP is actually fine
on MMIO, if a PCI BAR is passed through to a guest (and assuming there
are no ordering dependencies on the loads). It's the fact that it's
missing from stage2 page tables and causes a guest exit which is the
problem.

Other than lying to a compiler and *pretending* there are ordering
constraints, or explicitly using inline asm, how would a guest OS
*prevent* the compiler from using problematic instructions?

>  NISV is really only useful to run non hypervisor 
> enlightened guests. Did you encounter a real world OS that was causing 
> NISV faults? And if so, what was causing these? It's most likely a 
> driver or OS bug.

The Windows CRB TPM driver uses LDP. Even if it *were* forbidden by the
architecture, the track record of getting Microsoft to fix bugs *even*
when they are egregious violations of both specifications and common
sense is... poor.

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